<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script  type="text/javascript" src="jquery.js"></script>
</head>
<body>
<div>
    <input>
    <a href="#" id="changeImg">
        <canvas class="show-captcha" id="canvas" width="150" height="40"></canvas>
    </a>
</div>


<script>
    jQuery(function($){

        /**生成一个随机数**/
        function randomNum(min, max) {
            return Math.floor(Math.random() * (max - min) + min);
        }
        /**生成一个随机色**/
        function randomColor(min, max) {
            var r = randomNum(min, max);
            var g = randomNum(min, max);
            var b= randomNum(min, max);
            return "rgb(" + r + "," + g + "," + b + ")";
        }
        var code=drawPic();

        $("#changeImg").click( function(e) {
            e.preventDefault();
            code=drawPic();
        })
        /**绘制验证码图片**/
        function drawPic() {
            var canvas = document.getElementById("canvas");
            var width = canvas.width;
            var height = canvas.height;
            //获取该canvas的2D绘图环境
            var ctx = canvas.getContext('2d');
            ctx.textBaseline ='bottom';
            /**绘制背景色**/
            ctx.fillStyle = randomColor(180,240);
            //颜色若太深可能导致看不清
            ctx.fillRect(0,0,width,height);
            /**绘制文字**/
            var str ='ABCEFGHJKLMNPQRSTWXYZ123456789';
            var code="";
            //生成四个验证码
            for(var i=1;i<=4;i++) {
                var txt = str[randomNum(0,str.length)];
                code=code+txt;
                ctx.fillStyle = randomColor(50,160);
                //随机生成字体颜色
                ctx.font = randomNum(15,40) +'px SimHei';
                //随机生成字体大小
                var x =10 +i *25;
                var y = randomNum(25,35);
                var deg = randomNum(-45,45);
                //修改坐标原点和旋转角度
                ctx.translate(x, y);
                ctx.rotate(deg * Math.PI /180);
                ctx.fillText(txt,0,0);
                //恢复坐标原点和旋转角度
                ctx.rotate(-deg * Math.PI /180);
                ctx.translate(-x, -y);
            }

            /**绘制干扰线**/
            for(var i=0;i<3;i++) {
                ctx.strokeStyle = randomColor(40, 180);
                ctx.beginPath();
                ctx.moveTo(randomNum(0,width/2), randomNum(0,height/2));
                ctx.lineTo(randomNum(0,width/2), randomNum(0,height));
                ctx.stroke();
            }
            /**绘制干扰点**/
            for(var i=0;i <50;i++) {
                ctx.fillStyle = randomColor(255);
                ctx.beginPath();
                ctx.arc(randomNum(0, width), randomNum(0, height),1,0,2* Math.PI);
                ctx.fill();
            }
            return code;
        }
    });

</script>
</body>
</html>